Customizing the DataGridView to support expanding/collapsing (ala TreeGridView)

One of the first things that I wanted to customize with the DataGridView is to make it support hierarchical data

If you read my first blog post you'll find out that I'm not a developer (anymore). Even though I'm not a developer I still like to take features and customize them to do something really cool. As far as the DataGridView goes, customizing it to support hierarchical data is a much larger task since the structure of the DGV doesn't lend itself to having different column sets, so, I decided I'd settled for a tree like structure.

Think of a TreeView combined with a ListView and that is basically what I wanted to go with.

NOTE: This code is not supported by Microsoft and is provided "as-is". This code it not meant to show "best practices", but just showing a concept.

NOTE: This control is written to require Visual Styles to be enabled on your computer. You'll need to modify it if you want to run the TreeGridView without visual styles. Some people have been able to modify the code to run without Visual Styles. See this post for details.

Anyway. The basic part of creating a DataGridView that can expand and collapse is to dynamically add and remove rows. That was the easy part. To make this really usable and extendable, I decided to add a lot code and make this easier to use. Here are some details:

Design

I wanted to ensure that the design of the TreeGridView supported normal TreeView type properties and features, so creating necessary classes to create the “tree view” experience wa necessary (see object model for more details).

Custom Painting – Painting an image in a cell is easy, but ensuring that the text from the DataGridViewTextBoxCell didn’t overlap the image took a bit of work. Using the Padding feature of the DataGridViewCellStyle, I add padding to the left side of the cell to account for the text and the indentation. This padding affects the painting and behavior of the text box cell, so editing a text cell correctly positions the editing control.

Siting/Unsiting a node – Since I don’t want to set padding and styling except when a node is actually displayed. When the node is displayed or in the grid, it is sited. When the node is sited I set all the necessary properties.

Unbound – Since expanding and collapsing is based upon dynamically adding and removing rows from the grid, I decided that unbound mode would be the best way to go with this. I’ve hidden the “databinding” properties and the virtual mode property since this doesn’t support those features.

Edit Mode – One thing that I had to deal with is that double-clicking a cell enters edit mode. This double-click occurs regardless of the padding, so double-click on the +\- symbol causes the control to enter edit mode. Edit also enters if you single click on a cell that already has focus. So, to deal with this I turn edit mode to be enabled only through programmatic means. I have code to handle the F2 key to enter edit mode. There are other ways to solve this, but I went with the F2 approach.

Object model structure

TreeGridNode - Just like a tree view, I wanted to have the concept of a node. I made the nodes class derive from a DataGridViewRow since a node in the list is the same as a row, just with a bit more info.

Here are some properties:

Nodes – Again, like the treeview, a node has children, so there is a Nodes property that returns child nodes. One of the challenges in coding this is to know when a node is actually a row or when it is just a node. A node is a row when it is in the grid, otherwise it is just a node.

IsSited – A node is “sited” when it is contained in the grid as a row. The Sited property is true in this case. There are a set of protected virtual methods on the TreeGridView class (SiteNode and UnSiteNode).

ImageIndex – Image index for the node’s image. Only used when an ImageList is associated with the TreeGridView.

Image – Image associated with the node. Sets or gets the image. When an ImageList is associated with the TreeGridView and an ImageIndex is set then this returns an image from the ImageList. You can set the Image property to an image if you aren’t using an ImageList.

Cells – Returns the cells for the given node. This wasn’t easy to do since accessing the cells for a node (or row) when the node isn’t sited. Using the DataGridView’s CreateCells method I can get the correct cells collection when the node isn’t in the grid.

TreeGridCell/Column – This is a special DataGridView cell that derives from the DataGridViewTextBoxCell. The main thing that this custom cell class does is to customize the cell drawing to make it look like a tree node. That means that it draws the node’s image and the +/- icons and the tree lines. The custom cell also is where a node detects when you click the mouse to expand or collapse a node. NOTE: A lot more work can be done to correctly detect that the mouse is directly over the +/- image. Right now I’m not doing that.

TreeGridView – This class derives from the DataGridView control. Many things are done in this class. Nodes are sited/unsited in the grid as actual rows. Somce DataGridView Properties are hidden since they do not apply.

Here are some properties:

VirtualNodes – One of the common things done with a normal TreeView is to dynamically add child nodes when the user is expanding the parent. With the normal TreeView usres add temp child nodes to get the + sign and support expanding, then remove the temp node later. With the VirtualNodes property, the TreeGridView always displays a + sign next to a node, even if the node doesn’t have any children. Then, by handling the Expanding event you can dynamically add child nodes.

ImageList – ImageList associated with the TreeGridView

Nodes – Identifies the root nodes.

ShowLines – Determines if the TreeGridView shows lines between nodes.

CurrentNode – Identifies the node that has focus.

Here are some events:

Expanding – Again, like the treeview, this event occurs before a node is starting to expand. You can add nodes in this event to fill in more details of a child node collection. When the VirtualNodes property is true, the TreeGridView will display the node with a + sign next to it even when it doesn’t have any children. Handling the Expanding event is where you would dynamically add new child nodes.

Expanded – After a node is expanded.

Collapsing – Before a node is being collapsed

Collapsed – After a node has been collapsed.

I’ve included a simple test app that creates a news group reader looking list that supports expanding and collapsing.

Nice. Hierchical grids (or multi-column trees, depending on how you look at it) are really useful UI elements. It's frustrating that MS never provides one, forcing us to use 3rd party controls. This one looks promising.

ZAW

9 Jan 2006 11:57 PM

Thanks for your sharing.It is quite good.
Ah.. I got errors and I can't compile and reuse.
My VS version is professional version.Framework version is 2.0.50727
Can you help me?
regards
Zaw

Joo Lee

10 Jan 2006 2:52 PM

Mark - thanks for the great work. I think that in order to truly leverage the power of the datagridview in a hierarchical structure, we would need to incorporate more of the functions of both the DataGridView and the TreeView into your control, namely editable databound fields and restructuring/reordering of the nodes themselves. Such a control would be immensely effective in complex hierarchical data operations, such as structuring a Bill of Materials or orders/invoices. It's a great start though!

Ankur Gurha

13 Jan 2006 8:19 AM

To be honest, i just started looking to build a similar functionality today in one of our projects and here u go such a brilliant tip. Would let u updated how well did i manage.

I want to move focus to next cell. I am using DataGridView control for DataEntry. So, When user done with editing particular cell and press enter, focus should go on to the next cell (not next row). How could I do this? please help me.. I am looking for this from last 10 days. Not got the solution yet. My email id is: logan@fissionvector.com